You MUST have a 10-man royal rumble on any difficulty for this. There are no achievement stacks on this, so you will NOT be able to make a 30-man royal rumble on legend difficulty and get this achievement.
The best method I found was to use the Great Khali, since he is an Ultra Heaviweight and can eliminate many people when the opponents are hanging on the apron. Another good tip is to always carry a stored finisher, so if you hang off the apron and you are being pushed off (the elimination bar), you can hit and you will automatically enter the ring. After you win, you will earn this achievement.